1. Field of the Invention
The present invention relates to a method for electrically energizing and heating a platinum or platinum-alloy composite tube structure (hereafter referred to as “the electrical heating method according to the present invention”). In the present invention, the platinum or platinum-alloy composite tube structure includes a structure having two main tubes and a branch tube coupling the main tubes. The two main tubes and the branch tube in the composite tube structure are hollow tubes made of platinum or a platinum alloy. The composite tube structure is used as a conduit for molten glass in a glass manufacturing apparatus, such as a vacuum degassing apparatus. The present invention relates to a method for electrically energizing and heating the composite tube structure, and more specifically, to a method for electrically energizing and heating the branch tube in the composite tube structure.
The present invention also relates to a glass manufacturing method using the electrical heating method according to the present invention.
2. Discussion of Background
Glass manufacturing apparatuses use hollow tubes made of platinum or a platinum alloy, such as a platinum-gold alloy and a platinum-rhodium alloy, as conduits in which high-temperature molten glass flows. The conduits for molten glass include, for example, a tube provided to remove impurities from a glass manufacturing apparatus, a tube provided to supply molten glass from a glass manufacturing apparatus to a mold for forming an optical part, such as a lens or a prism, and a conduit from a melting tank to a forming tank.
In such a glass manufacturing apparatus, the conduits for molten glass are heated 50 that there will be no difference in temperature between the conduits and the molten glass flowing in them. The conduits may be heated externally by heat Sources, such as heaters, but many platinum or platinum-alloy hollow tubes are equipped with electrodes so as to be electrically energized and heated. JP-A-11-349334 discloses a heating apparatus for platinum tubes that can be used as conduits for molten glass.
A composite tube structure 100 shown in FIG. 3 may be used as a conduit for molten glass in a glass manufacturing apparatus. The composite tube structure 100 shown in FIG. 3 includes two main tubes 101 and 102 and a branch tube 103 connecting the main tube 101 and the main tube 102. In order that the main tube 101 of the composite tube structure 100 shown in FIG. 3 is electrically energized and heated, the main tube 101 may have an electrode 200 provided at each of an upper end and a lower end (not shown in the figure) thereof and have the electrodes 200 connected to an external power supply (not shown in the figure) for electrical energization and heating. Similarly, in order that the main tube 102 is electrically energized and heated, the main tube 102 may have an electrode 201 provided at each of an upper end and a lower end (not shown in the figure) thereof and have the electrodes 201 connected to an external power supply for electrical energization and heating.
The branch tube 103 is electrically energized and heated through the main tubes 101 and 102 because of having both ends coupled to the main tubes 101 and 102. More specifically, the branch tube 103 is electrically energized and heated by providing the electrodes 200 and 201 for electrical energization and heating, on the main tubes 101 and 102, connecting the electrodes 200 and 201 to an external power supply (not shown in the figure), and passing a current along an energizing path 300.
When the branch tube 103 is electrically energized and heated in the composite tube structure 100 shown in FIG. 3, energization control must be performed to prevent the branch tube 103 from being locally heated. If electricity is applied along the energizing path 300 in the composite tube structure 100 shown in FIG. 3, a current flows through the shortest path due to its nature. In a junction 104 between the main tube 101 and the branch tube 103, a current concentrates at a corner 104a included in the shortest current path. Similarly, in a junction 105 between the branch tube 103 and the main tube 102, a current concentrates at a corner 105a included in the shortest current path. The corners 104a and 105a where a current concentrates might be locally heated. If the corner 104a or 105a is locally heated, the corner 104a or 105a may be damaged by thermal stress. In addition, the characteristics of molten glass flowing in the composite tube structure 100 could be altered. Therefore, when the branch tube 103 is electrically energized and heated, energization control must be performed to prevent local heating from occurring at the corners 104a and 105a. 
When the branch tube 103 is electrically energized and heated, temperature monitoring may be performed at the corners 104a and 105a and at a part other than the corners of the branch tube 103 (hereafter sometimes referred to as the other part), such as a part near the longitudinal center of the branch tube 103, and energization control may be performed in accordance with a difference in temperature between the corners 104a and 105a and the other part. Any difference in temperature between the corners 104a and 105a and the other part indicates that the corners 104a and 105a have been locally heated. In such a case, energization control should be performed to eliminate or reduce the local heating of the corners 104a and 105a. The local heating of the corners 104a and 105a can be avoided or reduced by reducing the electrical heating of the branch tube 103. In the energization control performed for that purpose, either or both of a current flowing there and a voltage applied there should be reduced.
The inventors, however, have found that the electrical heating of the branch tube 103 may be performed inappropriately through the energization control described immediately above (hereafter referred to as the conventional energization control, performed in accordance with a difference in temperature between the corners 104a and 105a and the other part, while, in the hollow tube structure including the main tubes and the branch tube as shown in FIG. 3, electricity is applied between the upper ends of the main tubes or the lower ends of the main tubes).
When the energization control is based on a difference in temperature between the corners 104a and 105a and the other part, the energization control is actually performed in accordance with either a temperature difference ΔT1 between the corner 104a and the other part or a temperature difference ΔT2 between the corner 105a and the other part. Since the purpose of the energization control is to prevent the corners 104a and 105a from being locally heated, the energization control is normally performed in accordance with the larger temperature difference, ΔT1 or ΔT2.
However, the temperature differences ΔT1 and ΔT2 do not necessarily show the same tendency. For example, there may be ΔT1 alone, or there may be ΔT2 alone. Alternately, even with both temperature differences ΔT1 and ΔT2 being present, the two values may significantly differ from each other.
The temperature differences ΔT1 and ΔT2 show different tendencies when temperature rises are caused in different manners at the corners 104a and 105a. If the main tubes 101 and 102 have different diameters, different thicknesses, or different materials, the temperatures of both corners 104a and 105a would rise in different manners even when both tubes are energized through the same energizing path. If a heat source, such as a heater, is close to either the corner 104a or the corner 105a, the temperatures of both corners 104a and 105a would rise in different manners. Therefore, ΔT1 and ΔT1 show different tendencies in some cases.
If the temperature differences ΔT1 and ΔT2 show different tendencies, there is a possibility electrical energization and heating are inappropriately applied to the branch tube 103 by the conventional energization control. The conventional energization control is performed in accordance with the larger temperature difference, ΔT1 or ΔT2, as described earlier. If there is ΔT1 alone, the energization control would be performed in accordance with ΔT1. The presence of ΔT1 means that the corner 104a has been locally heated. The energization control based on ΔT1 is performed to reduce the electrical heating of the branch tube 103 so that the local heating of the corner 104a is eliminated. Because ΔT2 is not present in this case, the corner 105a is not locally heated. Under such circumstances, the energization control based on ΔT1 excessively reduces the electrical heating of the corner 105a. This leads to an is increase in the time required to heat the corner 105a to a desired temperature by electrical energization. At worst, it may be impossible to electrically energized and heat the corner 105a to the desired temperature. If the corner 105a cannot be electrically energized and heated to the desired temperature, there is a possibility that bubbles or deflected streams are formed in molten glass flowing in the composite tube structure 100, adversely affecting the quality of the molten glass.